RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option Summarized

1.   NSSAs are similar to the existing OSPF stub area configuration option but have the additional capability of importing AS external routes in a limited fashion.

2.   The original definition of the OSPF protocol imposes topological limitations that restrict simple cloud topologies from becoming OSPF stub areas. In particular, it is illegal for a stub area to import routes external to OSPF.

3.   This document describes a new optional type of OSPF area, somewhat humorously referred to as a “not-so-stubby” area (or NSSA), which hasthe capability of importing external routes in a limited fashion.

4.   The OSPF specification defines two general classes of area configuration. The first allows Type-5 LSAs to be flooded throughout the area.  In this configuration, Type-5 LSAs may be originated by routers internal to the area or flooded into the area by area border routers. These areas, referred to herein as Type-5 capable areas (or just plain areas in the OSPF specification), are distinguished by the fact that they can carry transit traffic.  The backbone is always a Type-5 capable area.  The second type of area configuration, calledstub, does not allow Type-5 LSAs to be propagated into/throughout the area and instead depends on default routing to external destinations.

5.   NSSAs are defined in much the same manner as existing stub areas. Tosupport NSSAs, a new option bit (the “N” bit) and a new type of LSA (Type-7) are defined. The “N” bit ensures that routers belonging toan NSSA agree on its configuration.  Similar to the stub area’s useof the “E” bit, both NSSA neighbors must agree on the setting of the”N” bit or the OSPF neighbor adjacency will not form.

6.    Type-7 LSAs provide for carrying external route information within anNSSA.

7.   There are two major semantic differences betweenType-5 LSAs and Type-7 LSAs.

            a.  Type-7 LSAs may be originated by and advertised throughout anNSSA; as with stub areas, Type-5 LSAs are not flooded intoNSSAs and do not originate there.

            b.  Type-7 LSAs are advertised only within a single NSSA; they arenot flooded into the backbone area or any other area by border routers, though the informationthat they contain                      may bepropagated into the backbone area.

8.   In order to allow limited exchange of external information across an NSSA border, NSSA border routers will translate selected Type-7 LSAsreceived from the NSSA into Type-5 LSAs.  These Type-5 LSAs will be  flooded to all Type-5 capable areas.

9.   NSSA border routers may beconfigured with address ranges so that multiple Type-7 LSAs may beaggregated into a single Type-5 LSA.

10.  In addition, an NSSA border router should originate a default LSA (IP network is 0.0.0.0/0) into the NSSA.  Default routes are necessarybecause NSSAs do not receive full routing information and must have a  default route in order to route to AS-external destinations.

11.   Note that a Type-7 default LSA originated by an NSSA border router is never translated into a Type-5 LSA, however, a Type-7 default LSAoriginated by an NSSA internal AS boundary router (one that is not an NSSA border router) may be translated into a Type-5 LSA.

12.  When summary routes are notimported into an NSSA, the default LSA originated into it by its border routers must be a Type-3 summary-LSA.  This default summary-LSA insures intra-AS connectivity to therest of the OSPF domain, as its default summary route is preferred over the default route of a Type-7 default LSA.  Without a defaultsummary route the OSPF domain’s inter-area traffic, which is normallyforwarded by summary routes, might exit the AS via the default route of a Type-7 default LSA originated by an NSSA internal router. TheType-7 default LSAs originated by NSSA internal routers and the no-summary option are mutually exclusive features. When summary routes are imported into the NSSA, the default LSA originated by a NSSA border router into the NSSA should be a Type-7 LSA.

13.    NSSA border routers may aggregate Type-7 routes by advertising a single Type-5 LSA for each Type-7 address range.  The Type-5 LSA resulting from a Type-7 address range match will be distributed toall Type-5 capable areas.

14.   A Type-7 address range includes the following configurable items.

            a. An [address,mask] pair.

            b. A status indication of either Advertise or DoNotAdvertise.

            c. An external route tag.

16.  Type-7 LSAs are identical to Type-5 LSAs except for the following:

a. The Type field in the LSA header is 7.

b. Type-7 LSAs are only flooded within the originating NSSA. The flooding of Type-7 LSAs follows the same rules as the flooding of Type-1 and Type-2 LSAs.

c. Type-7 LSAs are only listed within the OSPF area data structures of their respective NSSAs, making them area specific. Type-5 LSAs, which are flooded to all Type-5 capableareas, have global scope and are listed in the OSPF protocol data structure.

d. NSSA border routers select which Type-7 LSAs are translatedinto Type-5 LSAs and flooded into the OSPF domain’s transit topology.

e. Type-7 LSAs have a propagate (P) bit that, when set, tells an NSSA border router to translate a Type-7 LSA into a Type-5 LSA.

f. Those Type-7 LSAs that are to be translated into Type-5 LSAsmust have their forwarding address set.

17.  When an NSSA border router originates both a Type-5 LSA and a Type-7LSA for the same network, then the P-bit must be clear in the Type-7 LSA so that it isn’t translated into a Type-5 LSA by another NSSAborder router.  If the border router only originates a Type-7 LSA, it  may set the P-bit so that the network may be aggregated/propagated during Type-7 translation.  If an NSSA’s border router originates a Type-5 LSA with a forwarding address from the NSSA, it should alsooriginate a Type-7 LSA into the NSSA.  If two NSSA routers, both reachable from one another over the NSSA, originate functionallyequivalent Type-7 LSAs (i.e., same destination, cost and non-zeroforwarding address), then the router having the least preferred LSA should flush its LSA. Preferencebetween two Type-7 LSAs is determined by the following tie breakerrules:

                        a. An LSA with the P-bit set is preferred over one with the P-bitclear.

                        b. If the P-bit settings are the same, the LSA with the higherrouter ID is preferred.

18.  NSSA border routers must originate an LSA for the default destination into all their directly attached NSSAs in order to support intra-ASrouting and inter-AS routing. This default destination is advertised in either a Type-3 LSA or a Type-7 LSA.

19.  Since a Type-7 LSA only has area-wide flooding scope, when itsforwarding address is set to 0.0.0.0, its ASBR’s routing tableentry must be chosen from the originating NSSA.

20.  If the forwarding address is non-zero look up the forwarding address in the routing table.  For a Type-5 LSA the matchingrouting table entry must specify an intra-area or inter-areapath through a Type-5 capable area.  For a Type-7 LSA thematching routing table entry must specify an intra-area path through the LSA’s originating NSSA.  If no such path existsthen do nothing with this LSA and consider the next in thelist.

21.  Preference is defined as follows:

(a) Intra-area and inter-area paths are always preferred overAS external paths. 

(b) Type 1 external paths are always preferred over type 2 external paths. When all paths are type 2 external paths,the paths with the smallest advertised type 2 metric arealways preferred. 

(c) If the new AS external path is still indistinguishablefrom the current paths in N’s routing table entry, and RFC1583http://tools.ietf.org/html/rfc1583 Compatibility is set to “disabled”, select the preferred paths based on the intra-AS paths to the ASBR/forwarding addresses. Here intra-NSSA paths are equivalent to the intra-area paths of non-backbone regular OSPF areas. 

(d) If the new AS external path is still indistinguishablefrom the current paths in N’s routing table entry, selectthe preferred path based on a least cost comparison. Type1 external paths are compared by looking at the sum of thedistance to the ASBR/forwarding addresses and the advertised type 1 metric (X+Y).  Type 2 external pathsadvertising equal type 2 metrics are compared by looking at the distance to the ASBR/forwarding addresses.[OSPF]

(e) If the current LSA is functionally the same as an installed LSA (i.e., same destination, cost and non-zeroforwarding address) then apply the following priorities indeciding which LSA is preferred:

                         1. A Type-7 LSA with the P-bit set.

                         2. A Type-5 LSA.

             3. The LSA with the higher router ID.

22.  Type-7 Translator Election:

It is not recommended that multiple NSSA border routers performType-7 to Type-5 translation unless it is required to route packetsefficiently through Area 0 to an NSSA partitioned by Type-7 addressranges.  It is normally sufficient to have only one NSSA borderrouter perform the translation.  Excessive numbers of Type-7translators unnecessarily increase the size of the OSPF link statedata base.

The Type-7 LSA options field:

E-bit:Type-5 AS-external-LSAs are not flooded into/through OSPFstub areas and NSSAs. The E-bit  ensures that all membersof a stub area or NSSA agree on that area configuration.The E-bit is meaningful only in OSPF Hello and DatabaseDescription packets.  When the E-bit is clear in the Hellopacket sent out a particular interface, it means that therouter will neither send nor receive Type-5 AS-external-LSAs on that interface (in other words, the interfaceconnects to a stub area or NSSA).  Two routers will notbecome neighborsunless they agree on the state of the E-bit.

N-bit:The N-bit describes the router’s NSSA capability.  The N-bit is used only in Hello packets and ensuresthat all members of an NSSA agree on that area’s configuration.When the N-bit is set in the Hello packet that is sent outa particular interface, it means that the router will sendand receive Type-7 LSAs on that interface.  Two routerswill not form an adjacency unless they agree on the stateof the N-bit.  If the N-bit is set in the options field,the E-bit must be clear.

P-bit:The P-bit is used only in the Type-7 LSA header.  It flagsthe NSSA border router to translate the Type-7 LSA into aType-5 LSA.  The default setting for the P-bit is clear.

Router-LSAs :        

Router-LSAs are the Type-1 LSAs. Each router in an area originates arouter-LSA. The LSA describes the state and cost of the router’slinks (i.e., interfaces) to the area.  All of the router’s links to the area must be described in a single router-LSA.  In router-LSAs, the Link State ID field is set to the router’s OSPFRouter ID.  Router-LSAs are flooded throughout a single area only.

bit V:When set, the router is an endpoint of one or more fullyadjacent virtual links having the described area as their transit area (V is for virtual link endpoint).

bit E:When set, the router is an AS boundary router (E is for external).  ALL NSSA border routers set bit E in.Those router-LSAs originated into directly attached Type-5 capableareas. An NSSA’s AS boundary routers also set bit E in their router-LSAs originated into the NSSA.

bit B:When set, the router is an area border router (B is forborder).

bit W:When set, the router is a wild-card multicast receiver (W isfor wild).

bit Nt:When set, the router is an NSSA border router that isunconditionally translating Type-7 LSAs into Type-5 LSAs (Nt  stands for NSSA translation).  Note that such routers havetheir NSSATranslatorRole area configuration parameter set to Always.

Forwarding address:

Data traffic for the advertised destination will be forwarded tothis address.  If the forwarding address is set to 0.0.0.0, datatraffic will be forwarded to the LSA’s originator (i.e., theresponsible NSSA AS boundary router).  Normally the next hopaddress of an installed AS external route learned by an NSSA ASBRfrom an adjacent AS points at one of the adjacent AS’s gatewayrouters.  If this address belongs to a network connected to the NSSA ASBR via one of its NSSAs’ active interfaces, then it is theforwarding address for the route’s Type-7 LSA originated into thisNSSA.  For an NSSA with no such network the forwarding address iseither an address from one of its active interfaces or 0.0.0.0 If the P-bit is set, the forwarding address must be non-zero,otherwise it may be 0.0.0.0.

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